This invention relates generally to the interaction of the dispersal and mixing of one reactant stream into another. More specifically, this invention relates to the use of plate structures for distribution, contacting and mixing of reactants.
Certain chemical reactions are highly sensitive to the contacting conditions under which the reactants are brought together. Contacting conditions that can have a profound effect on the production of products in some reactions include physiochemical conditions such as reaction time, reagent concentration, reagent dispersion and temperature conditions. An example of a highly sensitive process of this type is the sulfonation of various compounds with a sulfonating agent. The initially formed sulfonates indicate a relatively high thermodynamic instability. It is well known that mild sulfonation conditions including short reaction times and low concentration gradients yield different products when compared to more drastic operating conditions.
A common method of controlling the contact between reactants in a reaction that is highly sensitive to process conditions is by the use of a thin film or falling film reaction zone. Falling film evaporators and reactors are well known in the art and are readily available commercially. Falling film evaporators pass a thin film of a liquid stream down one side of a heat exchange surface in indirect heat exchange with a heating medium that contacts an opposite side of the heat exchange surface and causes an at least partial evaporation of the falling liquid. Falling film reactors comprise a plurality of tubes or plates over which a thin film of one reactant is dispersed for counter-current or co-current contact with a gaseous reactant stream. In the case of evaporation or reaction laminar flow layers in the thin film can inhibit heat transfer and diffusion of vapor.
One of the most well known falling film reactor arrangements is for the continuous sulfonation or sulfation of fluid state organic substances by reaction with sulfur trioxide (sulfuric anhydride) (SO3). In traditional falling film arrangements, the SO3or other reactant is kept in a gaseous state. The reaction of the SO3 with the organic substances is strongly exothermic throughout the reaction which occurs rapidly or in many cases goes nearly instantaneously to completion. The gaseous SO3 is normally diluted with air or other inert gases to a reduced concentration of 4 to 15 wt-% which attenuates the severity of the reaction. The provision of cooling to the falling film contact surfaces also avoids the generation of temperature peaks from the highly exothermic reaction.
U.S. Pat. No. 3,925,441 issued to Toyoda et al. describes the use of flat plates for falling film sulfonation.
U.S. Pat. No. 5,445,801 to Pisoni describes a tube arrangement for falling film sulfonation that provides improved liquid distribution and accommodates expansion of the tubes.
U.S. Pat. No. 4,059,620 issued to Johnson, Jr. describes the advantages of maintaining a desired heat exchange profile during the sulfonation of organic compounds with sulfur trioxide.
The sulfonation or reaction of other organic compounds can cause extensive side reactions. Side reactions are best minimized by a uniform distribution of the falling liquid with gaseous reactants over the contact surfaces. Perhaps more important is the need to keep the sulfonating compound in relatively low concentration. Systems for controlling the distribution of liquid into tubes or plate arrangements for falling film reactors include weir and dam systems and slit or orifice arrangements that can be mechanically adjusted in various ways. Nevertheless, minor irregularities in the delivery systems to the top of the falling film apparatus can result in substantial flow variations with the attendant drawback of side reaction production. In addition to the problems associated with uniform delivery to a falling film contact surface, variations in the surface also create flow irregularities that can lead to non-uniform contacting and promote side reaction production.
The systems that use a gaseous phase reactant to contact the wetted walls of the falling film reactor also have the disadvantage or requiring a large circulation of gas in addition to the circulation of the liquid phase material down the walls of the reactor and the circulation of a cooling fluid. Care must be taken to control the concentration of the gaseous reactant in the gas phase. As a result the gas phase reactant is typically diluted with another gas to maintain a low reactant concentration and avoid unwanted by-product formation. For example in the sulfonation of aromatic hydrocarbons, a film of aromatic hydrocarbon is passed down the walls of channels through which an air stream containing dilute SO3 circulates. Supplying the air stream requires continual drying of large quantity of air if the air passes once through the channels. Recirculation of the air ordinarily necessitates purification to prevent product re-entrainment which will cause by-product formation.
The use of a permeable wall to introduce reagents into reaction zones is disclosed in U.S. Pat. No. 3,375,288. It is known to carry out a sulfonation reaction with liquid phase reagents in a reaction zone that has fluid permeable walls. An article entitled xe2x80x9cReactors for the 21st Centuryxe2x80x9d by Gerald Ondrey et al., Chemical Engineering, June 1996, pp. 39-45 and U.S. Pat. No. 5,583,240 discloses the passing of a sulfonation agent through permeable tubes that are surrounded by the sulfonation substrate. The tubes have a low permeability that maintains the sulfonating agent in low concentration. The tubes contain a packing of particulate material to provide the required good mixing of the sulfonating agent that permeates the tube wall.
A reactor system is sought that will eliminate the need for diluent gas addition or recirculation, reduce boundary layer limitations in the dispersion of a reagent in low concentrations in a liquid contactors, overcome any initial mal-distribution of liquid reactants in a liquid phase contactor, avoid the need for internal packing and facilitate the control of reaction temperature by promoting indirect heat transfer.
Accordingly, an object of this invention is to provide an apparatus and process for a fluid contactor that continually redistributes reactants.
Another object of this invention is to provide a fluid reactant contactor that facilitates good distribution and dispersion while promoting thorough mixing of the dispersed compound into a reagent.
A yet further object of this invention is to provide good distribution and dispersion of reaction fluids while simultaneously facilitating indirect heat exchange.
These and other objectives are achieved by an arrangement for a fluid distributor contactor-type reactor that uses plates containing permeable or perforated portions. The arrangement circulates the two reactants in alternate channels defined by spaces between parallel stacked plates. One reactant enters one set of channels that serve as reaction channels. A set of second channels, interleaved with the reaction channels, serve as distribution channels that can also provide a heat exchange function. Finely dispersed openings or permeation sites in the perforated plates distribute one reactant at low concentration from the distribution channels into the reaction channels. Dispersal of the reactant through the plate will introduce turbulence and promote good mixing of the reactants in the reaction channels. The pattern and size of the holes or permeable sections on the perforated plates may be varied as desired to disperse a carefully controlled amount of fluid across the plates over a large surface area. By maintaining a low addition rate of injected fluid reactant over the contact area, the concentration of the added reactant in the reaction channels may be kept as low as desired. Pressure drop across the perforated plate may be controlled to attain the desired degree of reactant penetration into the reaction channels. In liquid phase systems the addition of the reactant directly into the reaction channels eliminates the gas medium and any need for the associated recycle, separation or drying that were part of falling film contacting.
It is also useful to provide the perforated plates with a form or projections that increase the turbulence in the reaction channels. Such forms can include pins, rods or tabs extending outward from the plates to mildly agitate the flow through the reaction channels. Such flow agitation should be kept below a level that will cause substantial variation in the residence time of reactant through the reaction channels. A preferred plate form for introducing turbulence use corrugated plates that are stacked next to or in close proximity to each other to create the reactant and distribution flow channels.
The distribution channels that supply the liquid reactant to the perforations can also provide the passageways for indirect heat exchange of the reactant and product fluid in the reaction channels with the fluid in the distribution channels on the opposite side of the plates. A large excess volume of the second reactant normally circulates through the heat exchange channels to provide sufficient heating or cooling with only a small amount of the fluid reactant passing through the perforations for reaction in the reaction channels. The reactant is preferably circulated through the distribution channels at a much higher circulation rate than the dispersal rate of fluid across the reaction channels. Thus the reactant circulates at a high rate in the distribution channels as an optional indirect heat transfer fluid. Preferably the perforated plates will have a shape or form that promotes a high degree of heat transfer between the distribution and the reaction channels. Thin wall, relatively flat plates provide the best heat transfer characteristics. Again turbulence introducing forms or structures for the perforated plates are again beneficial for the heat transfer as well as the mixing functions. Corrugated plates are the preferred turbulence introducing form for both heat transfer and mixing. The injection of the reactant fluid from the heat exchange channels when performed with sufficient pressure drop provides velocity that also aids in creating turbulence.
The corrugations on the preferred form of the plates can be varied to suit the fluid flow properties of the fluid and in particular may be varied over the height of the contacting zone to vary fluid residence time and turbulence over different parts of the plates. The corrugated plates may be spaced apart to increase the flowing volume in either the reactant or distribution channels but preferably make contact with each other to provide structural stability. Turbulence introduced by the corrugated plates will again facilitate indirect heat transfer between the reactants in the distribution and reaction channels. In this manner the corrugated plate arrangement provides advantages for the dispersion, the contacting, the mixing and the heating or cooling of fluids in the reaction channels and in and between the reaction and distribution channels.
Accordingly, in a broad process embodiment, this invention is a process for the reaction of a fluid stream by the controlled addition of a fluid reactant. The process passes a first stream comprising a reactive fluid into a plurality of reaction channels defined by a first side of a plurality of stacked plates. A second stream comprising a reactant fluid circulates through a plurality of distribution channels defined by a second side of the plates to supply a reactant fluid and optionally to provide indirect heat exchange with the reactive fluid. Permeable portions distributed over the surface of the plates to control the contact of the reactant fluid with the reactive fluid distribute a portion of the reactant fluid into the reaction channels. The process recovers a reaction product from the reaction channels.
More specific process embodiments deal with the manner of distributing fluid and the specific fluid components and reactions. One such reaction is the sulfonation of a substrate with sulfur trioxide. Another reaction could be the contacting of a subcooled ethylene oxide-containing liquid with an organic material to perform ethoxylation.
In an apparatus embodiment, this invention is a reactor for the controlled distribution of reactants. The reactor includes a plurality of contacting plates containing perforations or permeable sections, stacked adjacent to one another to define reaction channels between the first sides of adjacent plates. Means are provided for passing a first fluid into the reaction channels. A distribution channel located between each reaction channel and defined by the second side of the plates distribute a second fluid through the plates and optionally circulate the second fluid as an indirect heat exchange medium. Means are provided for supplying the second fluid to the distribution channels and for collecting a fluid stream containing a reaction product from the reaction channels.
This invention may use any type of plate to define the alternate reaction and distribution channels. Preferred plates for this invention are those that will enhance the distribution of the fluid reactant that is injected through the plates into contact with the other liquid stream. The plate surface can enhance the intermixing of components by introducing additional turbulence to the surface of the plate defining the reaction channels. The turbulence should be enough to intermix the different fluids but not so great as to cause extensive backmixing of the fluids that can lead to non-uniform contacting and by-product generation. Corrugated plates with corrugations extending transverse to the direction of fluid circulation can be particularly beneficial in this regard.
In particular, corrugations on the contacting plates can be varied to suit the particular characteristics of the process and fluids employed. For low surface tension and low viscosity fluids, a relatively horizontal and shallow pitched corrugation is most beneficially employed. A slight downward pitch may be provided on the horizontal corrugations to provide a transverse movement when a liquid phase is present. The corrugation sections are preferably in a herring bone pattern so that the fluid flows back and forth in a horizontal direction across the reactor as it moves downwardly over the reactor thereby increasing the redistribution and uniformity of the flow. The number and height of corrugation rows can be varied in order to increase the dispersion of the fluids passing along the corrugations. In the case of liquids, as the viscosity of the liquid reactants increases, the slope of the corrugations and depth of the corrugations may be increased to provide additional redistribution and turbulence.
Controlled addition of the fluid reactant into the reaction channel is accomplished by distributing a portion of the fluid from the distribution channels across the plates through permeable sites that extend over a large area of the plates. When the contactor is used to provide an indirect heat exchange function, a relatively small amount of the fluid circulating in the distribution channels, usually less than 10% of the total circulating fluid, will normally pass through the permeable portions of the plate into contact with the fluid in the reaction channels films. More typically the amount of fluid passing through the plates will be less than 5% of the total circulating fluid entering the distribution/heat exchange channels.
Any type of structure may be used to provide the permeable sections of the plates. In simplest form the plates will contain a widely dispersed array of relatively fine perforations. The size and number of the perforations will of course depend on the fluid properties at the desired operating conditions. Fluid phase will typically be the most important fluid property. The invention can use all liquid phase fluids all gas phase fluids or may inject a liquid phase into a gas phase or a gas phase into a liquid phase. Other important fluid properties are the desired concentration of the fluid in the reaction channels and the pressure drop across the plates. In reactions such as sulfonation the perforations will ordinarily be in a size range of from 0.1 mm to 1 mm and will create about 5 to 10% open area across the plates. It may be desirable to decrease the density or size of perforation in the lower portions on the plates as more of the fluid in the reaction channels has been converted to product and need for additional reactant injection diminishes.
The multitude of perforations can have the added advantage of again introducing a desired degree of turbulence. Pressure drop across the plates may be regulated to futher control turbulence. A low pressure drop prevents the formation of extended fluid jets. Relatively high pressure drops are generally preferred to provide additional turbulence and further enhance mixing. High pressure drops may be particularly preferred where the fluid phases differ and a large jet may be desirable to force injected reactant gas across a liquid reactive stream. High pressure drop may also atomize a reactant liquid as it mixes with a reactive gas stream. Suitable pressure drops will vary widely depending of the fluid properties and the size of the perforations.
Suitable plate elements for this invention may also have a composite construction wherein a permeable material is incorporated onto the plates between the channels over holes in the plate. The permeable material can comprise an ultra-fine screen that inhibits any jet creation as liquid passes through the plate. Exposure of the circulating liquid to a fine screen material can introduce the desired turbulence and the well-dispersed area of permeable sections over the plate while the plate provides the necessary support of the screen material. Permeable membranes or other coatings across perforations would also provide a useful structure, especially for the control of gas phase flow, but are not preferred when performing simultaneous heat exchange since their insulating effects may interfere with the indirect heat transfer across the plate.
Additional details, embodiments, and arrangements of this invention are described in the following xe2x80x9cdetailed description of the invention.xe2x80x9d